Blance Terre
King Lancelot de la Blance Terre, Lancelot’s grandfather, loved the lady of the White Fortress, who was married to his cousin, the Duke of the White Fortress. When the duke discovered their love, he treacherously murdered King Lancelot – an act for which his castle became shrouded in darkness. The duke was crushed by a crumbling wall.
The darkness was to linger at the White Fortress until Galahad’s arrival years later.
Source
Vulgate Lancelot | 1215-1230